1. Prior Art
The present invention relates to an organic positive temperature coefficient thermistor that is used as a temperature sensor or overcurrent-protecting element, and has PTC (positive temperature coefficient of resistivity) characteristics or performance that its resistance value increases with increasing temperature.
2. Background Art
An organic positive temperature coefficient thermistor having conductive particles dispersed in a crystalline thermoplastic polymer has been well known in the art, as typically disclosed in U.S. Pat. Nos. 3,243,753 and 3,351,882. The increase in the resistance value is thought of as being due to the expansion of the crystalline polymer upon melting, which in turn cleaves a current-carrying path formed by the conductive fine particles.
An organic positive temperature coefficient thermistor can be used as a self control heater, an overcurrent-protecting element, and a temperature sensor. Requirements for these are that the resistance value is sufficiently low at room temperature in a non-operating state, the rate of change between the room-temperature resistance value and the resistance value in operation is sufficiently large, and the resistance value change upon repetitive operations is reduced.
To meet such requirements, it has been proposed to use a low-molecular organic compound such as wax and employ a thermoplastic polymer matrix for a binder. Such an organic positive temperature coefficient thermistor, for instance, includes a polyisobutylene/paraffin wax/carbon black system (F. Bueche, J. Appl. Phys., 44, 532, 1973), a styrene-butadiene rubber/paraffin wax/carbon black system (F. Bueche, J. Polymer Sci., 11, 1319, 1973), and a low-density polyethylene/paraffin wax/carbon black system (K. Ohe et al., Jpn. J. Appl. Phys., 10, 99, 1971). Self control heaters, current-limiting elements, etc. comprising an organic positive temperature coefficient thermistor using a low-molecular organic compound are also disclosed in JP-B's 62-16523, 7-109786 and 7-48396, and JP-A's 62-51184, 62-51185, 62-51186, 62-51187, 1-231284, 3-132001, 9-27383 and 9-69410. In these cases, the resistance value increase is believed to be due to the melting of the low-molecular organic compound.
One of advantages to the use of the low-molecular organic compound is that there is a sharp rise in the resistance increase with increasing temperature because the low-molecular organic compound is generally higher in crystallinity than a polymer. A polymer, because of being easily put into an over-cooled state, shows a hysteresis where the temperature at which there is a resistance decrease with decreasing temperature is usually lower than the temperature at which there is a resistance increase with increasing temperature. With the low-molecular organic compound it is then possible to keep this hysteresis small. By use of low-molecular organic compounds having different melting points, it is possible to easily control the temperature (operating temperature) at which there is a resistance increase. A polymer is susceptible to a melting point change depending on a difference in molecular weight and crystallinity, and its copolymerization with a comonomer, resulting in a variation in the crystallographic state. In this case, no sufficient PTC characteristics are often obtained. This is particular true of when the operating temperature is set at 100.degree. C. or lower.
In the organic positive temperature coefficient thermistors set forth in the above publications, however, no sensible tradeoff between low initial (room temperature) resistance and a large rate of resistance change is reached. pn. J. Appl. Phys., 10, 99, 1971 shows an example wherein the specific resistance value (.OMEGA..multidot.cm) increases by a factor of 10.sup.8. However, the specific resistance value at room temperature is as high as 10.sup.4 .OMEGA..multidot.cm, and so is impractical for an overcurrent-protecting element or temperature sensor in particular. Other publications show resistance value (.OMEGA.) or specific resistance value (.OMEGA..multidot.cm) increases in the range between 10 times or lower and about 10.sup.4 times, with the room-temperature resistance being not fully decreased.
On the other hand, JP-A's 2-156502, 2-230684, 3-132001 and 3-205777 disclose an organic positive temperature coefficient thermistor using a low-molecular organic compound and a thermosetting polymer behaving as a matrix. Since carbon black, and graphite are used as conductive particles, however, the rate of resistance change is as small as one order of magnitude or less and the room-temperature resistance is not sufficiently reduced or about 1.OMEGA..multidot.cm as well. Thus, no compromise is made between the low initial resistance and the large rate of resistance change.
JP-A's 55-68075, 58-34901, 63-170902, 2-33881, 9-9482 and 10-4002, and U.S. Pat. No. 4,966,729 propose an organic positive temperature coefficient thermistor constructed solely of a thermosetting polymer and conductive particles without recourse to a low-molecular organic compound. In these themistors, either, no compromise is achieved between a room-temperature resistance of up to 0.1 .OMEGA..multidot.cm and a large rate of resistance change of 5 orders of magnitude greater, because carbon black, and graphite are used as the conductive particles. Generally, thermistor systems composed merely of a thermosetting polymer and conductive particles have no distinct melting point, and so many of them show a sluggish resistance rise in temperature vs. resistance performance, failing to provide satisfactory performance in overcurrent-protecting element, temperature sensor, and like applications in particular.
In many cases, carbon black, and graphite have been used as conductive particles in prior art organic positive temperature coefficient thermistors including those set forth in the above publications. A problem with carbon black is, however, that when an increased amount of carbon black is used to lower the initial resistance value, no sufficient rate of resistance change is obtainable; no reasonable tradeoff between low initial resistance and a large rate of resistance change is obtainable. Sometimes, particles of generally available metals are used as conductive particles. In this case, too, it is difficult to arrive at a sensible tradeoff between the low initial resistance and the large rate of resistance change.
In Japanese Patent Application No. 9-350108, the inventors have already come up with an organic positive temperature coefficient thermistor comprising a thermoplastic polymer matrix, a low-molecular organic compound and a conductive particle having spiky protuberances. This thermistor has a sufficiently low room-temperature specific resistance of 8.times.10.sup.-2 .OMEGA..multidot.cm, a rate of resistance change of eleven orders of magnitude greater between an operating state and a non-operating state, and a reduced temperature vs. resistance curve hysteresis. In addition, the operating temperature is 40.degree. C. to 100.degree. C. inclusive. When thermistors are used as protective elements for secondary batteries, electric blankets, heaters for lavatory seats and vehicle seats, etc., an operating temperature of 100.degree. C. or greater poses a potential danger to the human body. With the safety of the human body in mind, the operating temperature must be 100.degree. C. or lower. In recent years, organic positive temperature coefficient thermistors have been increasingly demanded as over-current protecting elements for portable telephones, personal computers, etc. In view of the temperature at which they are used, too, thermistors having an operating temperature from 40.degree. C. to 100.degree. C. are desired.
However, this thermistor is found to be insufficient in terms of performance stability, especially with a noticeably increased resistance at high temperature or humidity or upon exposure to on-off loading. This appears to be due to the segregation, etc. of the working or active substance, i.e., the low-molecular organic compound upon repetitive melting/solidification cycles during operation, which segregation is ascribable to the low melting point and low melt viscosity (about 2 to 10 mm.sup.2 /sec. at 100.degree. C.) of the low-molecular organic compound. This in turn causes a change in the crystallographic or dispersion state of the low-molecular organic compound and conductive particles, resulting in a performance drop. Such a performance stability problem is important to the low-molecular organic compound acting as the working substance. All currently available thermistors using low-molecular organic compounds as active substances, inclusive of those mentioned above, are still less than satisfactory in terms of performance stability. In some cases, the thermistor elements undergo deformation.
On the other hand, JP-A 5-47503 discloses an organic positive temperature coefficient thermistor comprising a crystalline polymer, for instance, polyvinylidene fluoride and a conductive particle having spiky particles, for instance, spiky Ni powders. U.S. Pat. No. 5,378,407, too, discloses a thermistor comprising filamentary nickel having spiky protuberances, and a polyolefin, olefinic copolymer or fluoropolymer. However, these thermistors are still insufficient in terms of hysteresis and so are unsuitable for applications such as temperature sensors, although the effect on the tradeoff between low initial resistance and a large resistance change is improved. This is because no low-molecular organic compound is used as a working or active substance. Another problem with these thermistors is that when they are further heated after resistance increases upon operation, they show NTC (negative temperature coefficient of resistivity) behavior that the resistance value decreases with increasing temperature. It is to be noted that the above publications give no suggestion about the use of a low-molecular organic compound at all. To add to this, these thermistors have an operating temperature in excess of 100.degree. C. Some thermistors disclosed in the above publications have an operating temperature of 60 to 70.degree. C., but their performance becomes unstable upon repetitive operations.
JP-A 5-198403 and 5-198404 disclose an organic positive temperature coefficient thermistor comprising a mixture of a thermosetting resin and conductive particles having spiky protuberances, and show that the rate of change resistance obtained is 9 orders of magnitude greater. However, when the room-temperature resistance value is lowered by increasing the amount of a filler, no sufficient rate of resistance change is obtained. Thus, it is difficult to achieve a tradeoff between low initial resistance value and a large resistance change. Also, the thermistors fail to show a sufficiently sharp resistance rise because of being composed of the thermosetting resin and conductive particles. The above publications, too, are silent about the use of a low-molecular compound.
Never until now is an organic positive temperature coefficient thermistor obtained, which shows satisfactory performance at an operating temperature of 100.degree. C. or lower and has performance stability.